1. Field of the Invention
This invention pertains to the field of oil recovery from subterranean reservoirs.
2. Discussion of the Prior Art
In the recovery of oil from reservoirs by water displacement the presence of a free gas saturation will increase oil recovery by an amount about equal to the irreducible gas saturation. The irreducible gas saturation is the lowest saturation the gas can be reduced to by encroachment of water. As oil is produced from some oil reservoirs, especially those with little or no water drive, the pressure in the reservoir is reduced and bubbles of gas evolve from the oil in the reservoir which provide the energy to force the oil to the surface. This gas being in the form of gas in the reservoir is called free gas. The amount of free gas present is called the free gas saturation. However, in reservoirs with a very active water drive where the water encroaches as fast as oil is produced the pressure in the reservoir may not decline appreciably as oil is produced. The encroaching water may keep the pressure essentially constant throughout the productive life of the reservoir, consequently no gas will evolve from solution in the oil. It is, therefore, an object of this invention to provide a method whereby gas evolution will take place in a reservoir with an active water drive and thereby increase the oil recovery by an amount equal to the irreducible gas saturation.